My attempts to make a rapid prototyping machine that I will use to make parts for a machine that will be able to make parts for a copy of itself.

Wednesday, 4 November 2009

No compromise extruder

I have settled on using vitreous enamel resistors embedded in an aluminium block for the heater. I think they are the easiest heater to make and likely to be the most durable. They also work fine with simple bang-bang control, whereas it would appear that the Nichrome and Kapton version requires PID.

One of the aims of my new design is to reduce the amount of molten plastic to minimise ooze. Also less molten plastic means less viscous drag. I also wanted to reduce the thermal mass (to reduce the warm up time) and completely cover the hot part with insulation to allow a fan to blow on the work-piece without cooling the nozzle.

To achieve these aims I switched to a smaller resistor (same resistance but less wattage) and mounted it horizontally rather than vertically. There is some risk that the resistor may fail but I think as long as it has good thermal contact with the aluminium block, so that its outside temperature is less than 240C, then I have a good chance it will last.

The smaller resistor also means a much smaller surface area so less heat is lost. T0 keep the molten filament path as short as possible I combined the heater and the nozzle and made it from one piece of aluminium. That also gives very good thermal coupling between the nozzle tip, the melt chamber, the heater and the thermistor.

I turned it out of a block of aluminium using my manual lathe and a four jaw chuck, but I think I could also mill it out of 12mm bar using HydraRaptor.

A feature that I have used on my previous extruders is to cover as much of the nozzle as possible with PTFE. That stops the filament sticking so that it can be wiped off reliably with a brush. It also insulates the nozzle.

My previous nozzle cap implementations have been turned from PTFE rod. The downside of that is that the working face, that has been cut and faced on the lathe, is not as smooth and slippery as the original stock.

To cover the face of this version I used a 3mm sheet of PTFE so it has the original shiny surface.

Normally PTFE is too slippery to glue so my original plan was to screw it on with some tiny countersunk screws. However, the sheet I bought was etched on the back to allow it to be glued, so I stuck it on with RTV silicone adhesive sold for gluing hinges onto glass oven doors.

To insulate the rest of the heater I milled a cover out of a slice of 25mm PTFE rod.

I normally stick items to be milled onto the back of a floor laminate off-cut using stencil mount spray. I didn't think that was going to work with a PTFE cylindrical slice that is only a little bigger than the finished item. Instead I milled a hole in a piece of 6mm acrylic sheet that was already stuck down with stencil mount. The hole was slightly smaller than the PTFE so I faced it and chamfered it on the lathe and then hammered it in.

I roughed the shape with a 1/8" end mill and then sharpened the internal corners and cut the slots for the resistor leads with a 1mm end mill. I tried to mill the whole thing with a 1mm bit but it snapped due to a build up of burr in the deep pocket. On reflection it was silly to expect to be able to mill deep pockets with a 1mm bit and of course it is much faster to rough it with a bigger bit.

I used my normal technique of taking 0.1mm depth cuts at 16mm. That allows me to mill plastic with no coolant, but I expect I could have made much deeper cuts in PTFE. It mills very nicely, probably because it is soft and has a high melting point and low friction.

I haven't done any milling for a long time so for anybody new to my blog here is my the milling set-up: -

It is simply a Minicraft drill with some very sturdy mounts. The spindle controller I made originally would need its micro replaced as the one I used has a bug in its I2C interface. Instead I just connected it to the spare high current output on my new extruder controller.

The remaining part of the extruder is the stainless steel insulator.

I made the transition zone shorter than the last one I made because I wanted all of the inside of the transition to be tapered. The aluminium sleeve carries away the heat from the cold end of the transition to an aluminium plate that forms the base of the extruder. That in turn carries the heat to the z-axis via an aluminium bracket. I used heatsink compound on the joints.

Here is a view of the bottom half of the extruder: -

And here is a cross section showing the internal details: -

So that was the plan, what could go wrong? Well everything really! The first problem was that the resistor shorted out to the aluminium block. The smaller resistor only has a thin layer of enamel over its wire. Normally I wrap aluminium foil round it to make it a tight fit. I didn't drill the hole big enough so it was a tight fit with only one layer and pushing it in abraded the enamel. The solution would be a bigger hole and more layers of foil, but I just glued it with Cerastil as a quick fix. Of course it only failed after I had fully assembled it and run some heat cycles so I had to strip it down again to fix it. Not easy once the wiring has been added.

The next problem is that it leaks. I think it is because I dropped the extruder when I was building it and bent the thin edge at the end of the stainless steel barrel. That forms the seal with the heater block, so even though I straightened it I think the seal is compromised. I keep tightening it and thinking it is fixed but after hours of operation plastic starts to appear at the bottom of the PTFE cover.

The other problem is that mostly it extrudes very well, I now do the outline at 16mm/s and the infill at 32mm/s, but sometimes the force needed to push the filament gets higher and causes the motor to skip steps, or the bracket to bend so far that the worm gear skips a tooth.

I have made several objects taking between one and two hours and it worked fine. Other times, mainly when I was making small test objects with Erik, it will completely jam. Actually it seems to jam when it is leaking badly, which implies the pressure of the molten plastic is much higher as well as the force to push the filament. The only explanation I can think of is there is an intermittent blockage of the nozzle exit. More investigation required.

@Zaggo, Yes it difficult to make even with a lathe because it is not round and has to be turned from both sides. To mill it it would have to also be done from both side requiring a jig or an edge finder.

I have another design where the nozzle is turned from hex brass stock and screws onto the pipe but also has an external thread that screws into the AL heater block. It comes out a bit bigger though and still needs 12 lathe operations.

@jglauche, Yes it a stainless 1/4" steel pipe with an ID of about 3.6mm. I tapped both ends 1/4" UNF which is very hard work even with a split die and cutting compound. I had to put flats on the blank bit in the middle to grip it on the lathe chuck.

Been thinking about your comments re airflow over the head, cooling and Adrians experiments with elevated ambient build temperatures. Couple all of that with attempts to manage a reduced thermal transition zone and give it a quick stir.

Is there perhaps some mileage in placing a tube shield/guide around your barrel and ducting heated 80 degree air around the barrel to keep it cool, pre warm the filament (without meting and overly softening it) and ultimately create a warmed zone at the point of extrusion (something like gas sheilds produced in the tig welding process)

You may need to add some thermal insulation around the top side of your extruder head/heater.

Overall though running at a consistently higher temperature than ambient will make the extruder performance predictable/consistent.

Forced cooling (to 80 degrees anyway) around your stem should allow you to keep that transition zone very small. Primarily as your cooling method would then be active rather than passive.

To tackle warping I will first add a heated bed. I may try blowing hot air past the nozzle as well. It will vastly increase the power consumption of the machine though as it is only 40W now and probaply needs 50W under the bed and a few hundred to blow air over the object.

I don't think preheating the filament is a good idea. I try to keep it less than 30C for PLA and it will jam if it gets to 50C. ABS has to be less than 80C for as long as possible to reduce the friction.

Great post! I hope the plugging problem will stop bugging you... Seeing that this design is so sound and has incorporates so many of your findings and good practices, I'm puzzled why this happens. Especially since you have a taper in the lower section. Any plug that would form and create friction will have the right shape to be easily forced into the hotter section. I've had my fair share of nozzle blocks, but it was usually the orifice with crumbly brown/oxidised ABS inside it, or a heater barrel with a plug. Pulling the filament out a few mm after turning the heater off seems to mitigate this for me.

@AKA74: Instead of pre-heating filament, a small aquarium pump might allow you to pre-cool instead, in order to get a short thermal zone. E.g. if you make a very small perforation near the hot zone and the cold air would escape through it (cool it from within the top section of the extruder). At that point the extruder would probably not start leaking because of the cooling. A single pump can be shared by multiple extruders. Such cooling would reduce the springiness of the filament when it's colder of a longer length allowing sharper stops and starts. The 'motor reversing' anti-ooze strategy turned out to completely eliminate ooze (even for PLA, which becomes liquid as honey), but reversing (slightly) adds to the build time, especially when there's a lot of complexity in the model, even with stepper motor based reversing (which can reverse some mm very rapidly). The GM3 or other gearbox based extruders (Makerbot :S ) (may) have too much momentum for quick pullback.Then again, it wouldn't make a radical difference in performance while adding quite some complexity.

I am thinking about supplying a controlled stream of hot air near the extrusion, though. I could further reduce warping and risk of delamination. Though I think that much of my delamination had to do with ooze which was supposed to be a structural part of the object: it requires just one weakest link/layer to break an object. Also, it would allow the extrusion temperature to be lower, keeping natural ABS more white instead of beige. Instead of using a separate heater, we could also use the extruders' heater for heating the object with hot air, by controlling the temperature via the air-flow rate. But this might affect the extruder temperature too much and cause problems there (resulting in an unstable setup). It might be best to reduce interactions between such critical modules to a minimum instead of saving on nichrome wire or resistors :)

Fantastic post very intesesting watching the evolution of your extruder from the original MK1 Darwin.

You stopped using PTFE as the temprateure insulator due to the way it manges to deform over time.I can't remember why you didn't use Peek as the insulator was it for a similar reason ?

My Intention for the extruder is to use the 1" long welding tip as the hot zone this is screwed into a 35mm x 16mm peak insulator. I guess I will have to have PID control for temprature.Do you think it would be better have a Peak insulator to slide over the nichrome & Kapton heated copper welding rod extruder section instead of using fiberglass wadding?

Also if make a cover like this would it be best having a tight fit or 1mm clearance from the Kapton taped element?

I made a standard extruder with PEEK and AL instead of PTFE and brass but it never worked reliably: hydraraptor.blogspot.com/2008/12/new-materialsl. It required too much force to extrude. The melt zone was too big and not slippery enough.

I also realised a taper was important so that means the insulator has to screw into the heater, not the other way round.

PEEK does creep somewhat as well. Stainless steel is self supporting and can be very thin so the heat loss is reasonable.

Having said all that my "no compromise design" is not reliable yet for reasons I don't understand, so I may be back to the drawing board again.

I cover the heater with PTFE because it is a reasonable insulator and very slipery. I don't find PEEK as good an insulator or as slipery. I can touch the PTFE cover momentarily without burning myself.

Fibreglass wool is a much better insulator but I don't like handling it. It is also more bulky and I like my extruder slender so it can get between objects.

My guess would be that an air gap would be beneficial as long as the air cannot escape at the top and convect. Good insulators use trapped air so that its poor conduction is utilised without it being able to convect.

Another technique I haven't tried is to cover the heater in RTV silicone.

nophead: I have thinked a lot lately about your design. I drawed my idea, what could be wrong:

http://sites.google.com/site/khiraly1/ultimate_extruder.png

I think the critical thing, is the cross section starts to shrink at the point where the two material (sainless steel - aluminium) are connected. I think it would be better, if after the connection the cross section would grow a little and only shrink after it. Dunno how can it made by a lathe.

What wattage resistor are you currently using? I'm thinking about making a design like this but I'm not sure what wattage will be good for a resistor. I'm fed up with the Makerbot extruder design, so I'm starting from scratch. Thanks!

The resistor is a 6R8RWM 4x10. It is rated at 3W @ 25C, but also 5.5W with surface temperature less than 450C, so I don't really understand the specs for these resistors.

I am running it at with 12V which gives 21W and about 41% PWM after it has reached temperature, which is 8.6W. I have printed a full Mendel with it and no sign of any problems so I am confident it will last at least as long as any home made nichrome arrangement.

Hi Nophead. I really like your resistor based extruder design. I had a go at winding Nichrome wire earlier today and it damn springy stuff! Just had a look to get myself some of these resistors, (although I'm running from a 24v supply so I'm planning on getting 22Ohm resistors), and the Farnell minimum order is £20. I have found these seemingly equivalent at Maplin though: http://www.maplin.co.uk/Module.aspx?ModuleNo=2173 so I'll be getting some in the morning and let people know if they are any good.

Having played around with these for a few days, I found out that it's possible to break off the casing revealing a cylindrically shaped bare wirewound resistor (approx 4.5mm dia x 15mm long). I managed to case the resistor in fire cement and push it into a 6mm dia hole in my heater block. It worked quite well. However, I have since found a good source of wirewound resistors on ebay (http://stores.ebay.co.uk/dannell-4you_Resistors-WireWound_W0QQ_fsubZ18541363QQ_sidZ550551596QQ_trksidZp4634Q2ec0Q2em322) although they don't seem to have the 6R8 resistor you're using, but 10Ohms works well for me (with 24v).

i have a big bronze heater block and it takes almost 8 mins to heat up bang mode so i am thinking of using 2 resistors, would 6R8 at 3W make any sense to use in paralel to make it heat faster? or should i get other values resistors? not sure how that will work out and basically not sure of the safety of the control fet. ty